Denture curing apparatus and method

ABSTRACT

An apparatus and method for curing light curable material using unfiltered light. A vented housing encloses a single lamp filament light source which emits visible light. The light source is positioned above and to the side of the light curable material which is supported on a rotating table during light curing. Light incident to the surface of the light curable material is at least about 50 milliwatts per square centimeter. The light curable material has a surface temperature of at least 150° F. (65.6° C.) after 2 minutes.

This is a continuation of application Ser. No. 07/693,212, filed Apr.30, 1991, now abandoned.

The invention relates to an apparatus and method for light curing oflight curable material to form dental restorations and prosthetic invitro. In particular the invention uses a single light source having afilament to cure light curable material. The apparatus of the inventionhas substantially reduced power consumption and equipment cost while atleast maintaining the curing temperature provided by prior art lightcuring apparatus. The invention is particularly and especially preferredfor use in the dental field in the construction and repair of dentalappliances, such as retainers and dental prosthetics, for exampledentures and bridges. The invention also has excellent characteristicsallowing its expediant use in special medical applications, such ascustom fabrication of hearing aids and finger splints.

Gonser et al in U.S. Pat. No. 4,546,261 disclose denture curingapparatus and method in which an array of light sources direct light inthe 400 to 500 nanometer wavelength range onto a denture. Tateosian etal in U.S. Pat. Nos. 4,863,977; 4,711,913 and 4,551,486 discloses aprocess for preparing interpenetrating polymer network objects. Thesecompositions are cured using light and/or heat. Such compositions havebeen cured in the four lamp denture curing apparatus of Gosner et al anda three lamp apparatus disclosed in Triad II VLC Light Curing Unit;Operation and service Manual from Dentsply, 1987; which is alsodisclosed by Gosner U.S. Pat. No. Des. 308,976 (hereinafter referred toas Triad II).

Some prior art denture curing apparatus provide inadequate irradiatepower to cure the light curable material. Prior art denture curingapparatus which do provide adequate irradiant power to cure lightcurable material use an array of tungsten filament light sources whichare expensive. The prior art does not disclose an apparatus for denturecuring which uses a single light source having a tungsten filament as isprovided by the present invention. These problems of the prior art areovercome by the denture curing apparatus and method of the invention.

It is an object of the invention to provide an apparatus for denturecuring using a single unfiltered light source having a tungsten filamentand enclosing halogen gas.

It is an object of the invention to provide an apparatus for denturecuring using a single unfiltered light source having a filamentpositioned within a vented housing.

It is an object of the invention to provide an apparatus for denturecuring using a single unfiltered light source having a filamentpositioned above a rotatable table within a vented housing.

It is an object of the invention to provide an apparatus for denturecuring which has reduced power consumption and reduced equipment costwhile at least maintaining the curing temperature provided by prior artlight curing apparatus.

It is an object of the invention to provide a method for curing denturesusing light by providing a housing, a single filament light source, andpolymerizable material in the form of a denture, and radiating lightfrom the light source onto the polymerizable material to form a cureddenture.

It is an object of the invention to provide a method for light curinglight curable material at more than 70 milliwatts per square centimeterfor at least 2 minutes while maintaining the material at least above150° F. (65.6° C).

The temperatures of the light curable material disclosed throughout thisspecification are surface temperatures which are generally uniform.Temperature measurements recited herein were made with a thermocoupleprobe having a digital readout.

Unfiltered light as used throughout this disclosure refers to lightwhich is not subjected to means for removing specific wave lengths.While the glass enclosing the filament may remove insignificant amountsof light, the light passing therethrough is considered to be unfiltered.

Actinic light as used herein refers to light capable of initiatingphotochemical reactions. The light sources used in accordance with theinvention provide actinic light.

The invention overcomes the problems of prior art light curing apparatusand methods through its useful, novel and nonobvious features. Theadvantages of the invention are not found in prior art light curingapparatus and methods.

BRIEF SUMMARY OF THE INVENTION

An apparatus and method for curing light curable material usingunfiltered light. A vented housing encloses a single lamp filament lightsource which emits visible light. The light source is positioned aboveand to the side of the light curable material which is supported on arotating table during light curing. Light incident to the surface of thelight curable material is at least about 50 milliwatts per squarecentimeter. The light curable material has a surface temperature of atleast 150° F. (65.6° C.) after 2 minutes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an apparatus for curing dentures inaccordance with the invention.

FIG. 2 is a top view of an apparatus for curing dentures in accordancewith the invention.

FIG. 3 is a bottom view of an apparatus for curing dentures inaccordance with the invention.

FIG. 4 is a schematic diagram of an electrical circuit of an apparatusfor curing dentures in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is now described in reference to the preferred embodimentshown in FIGS. 1-4 in which the same numerals refer to the same partsthroughout the figures. FIG. 1 shows denture curing apparatus 10 whichincludes housing 12 and incandescent lamp 14. Preferably the lamp 14includes a reflector 14A. Preferably the reflector has a surface whichis adapted to distribute light, for example, by including smooth,faceted or peened surfaces. Incandesoent lamp 14 has filament 15.Filament 15 is positioned within a transparent enclosure filled with ahalogen gas or mixture of gases which is primarily halogen.

Light curable material 16 is supported on rotatable table 18. Urethanedimethacrylate resin with visible light activator may be used as thelight curable material 16. The height of rotatable table 18 isadjustable by turning knob 20. Knob 20 is connected to a helical screwwhich turns through a helically threaded nut connected to a carriagewhich supports table 18.

Preferred light curable material for use in accordance with theinvention is disclosed in U.S. Pat. No. 4,551,486 particularly at fromcolumn 1, line 46 to column 10, line 4, which is incorporated herein byreference. Preferred monomeric species useful in light curable denturemaking material are multifunctional and include acrylic and lower alkylacrylic acid esters. In a preferred embodiment of the invention thelight curable denture making material includes crosslinked polymer,filler, and crosslinking agent.

Housing 12 has a base 22 and a top 23 as shown in FIGS. 1-3. Door 24 isconnected by hinge 25 to base 22. Door 24 has reflective inner surface26 which is adapted to reflect electromagnetic radiation from halogencontaining incandescent lamp 14 onto light curable material 16. Uppervents 28 in top 23 allow heated air to be pulled by a fan from curingchamber 30 to the outside of the housing 12. Lower vents 31 allow air topass through base 22 and enter curing chamber 30.

In use light curable material 16 is positioned within curing chamber 30.The width, depth and height of the curing chamber 30 are sufficient toallow dental restorations, prosthetic and appliances made of material 16to be positioned therein.

With more particular reference to FIGS. 1 and 4 it is seen that door 24in its closed position is adapted to press push button 32. When pushbutton 32 is pressed switch 32S is in its closed position. Switches 34Sand 36S are closed by pressing the power switch lever 34 and the cureswitch lever 36. This permits electrical current to be conducted fromcurrent source 42 through tungsten filament 15 which emits infrared,visible and ultraviolet electromagnetic radiation. The period for cureis selected by setting the timer knob 38. Table switch lever 40 isadapted to actuate switch 40S.

Electrical current source 42 is connected through line 44 to circuitbreaker 46. Circuit breaker 46 is connected through line 48 to switch34S. Connector 52 operates switch 54 with switch 34S. Switch 34S isconnected through line 56 to switch 32S. Switch 32S is connected throughline 60 to switch 36S. Switch 32S is connected through line 64 to switch40. Switch 40S is connected through line 68 to motor 70. Motor 70 isconnected to ground through line 72. Switch 36S is connected throughline 74 to timer circuit 76. Timer circuit 76 is connected through lines78 and 78A to variable resistor 80. Timer circuit 76 is connectedthrough line 82 to filament 15. Timer circuit 76 is connected to groundthrough line 84. Filament 15 is connected to ground through line 72. Fan86 is connected to switch 34S through line 88. Fan 86 is connected toground through line 90.

To cure light curable material 16 the dental technician places thematerial on table 18 and closes door 24 depressing push button 32, whichcloses switch 32S. The dental technician then depresses lever 34 whichcloses switches 34S and 54 which connects fan 86 to electrical currentsource 42. Then the dental technician depresses lever 40 which closesswitch 40S connecting motor 70 to current source 42. The motor 70 turnstable 18 which rotates material 16. Finally the operator rotates knob 38to set timer 76, and presses lever 36 closing switch 36S to conveyelectrical current to filament 15. The material 16 cures in from about 2to about 10 minutes at from about 120° to about 200° F. (93.3° C.) whilebeing rotated in front of the light emitted from lamp 14. When the curetime has lapsed the dental technician removes the material 16 from table18.

Preferably the filament is adapted to emit sufficient electromagneticradiation to provide irradiant power on the upper surface of the curingmaterial of at least 50 milliwatts per square centimeter. Morepreferably, the filament is adapted to emit sufficient electromagneticradiation to provide irradiant power incident on the upper surface ofthe curing material of from about 50 to about 100 milliwatts per squarecentimeter.

Preferably the filament includes tungsten and is adapted to providesufficient electromagnetic radiation to maintain the temperature oflight curable material positioned within said housing at between 120°and 250° F. (48.9° and 121.1° C.). In an embodiment of the invention thefilament is adapted to use at least 200 watts of power.

Preferably the gas used to enclose the filament is argon or a mixture ofargon and less than 1 percent by volume of other gases, such as bromineand nitrogen.

Preferably, the polymerizable material has a surface temperature of atleast 150° F. (65.6° C.) after at least 2 less than 70 milliwatts persquare inch of incident light. More preferably, the polymerizablematerial has a surface temperature of at least 145° F. (62.7° C.) after10 minutes at less than 80 milliwatts per square inch of incident light.

Preferably the light source is positioned at an angle of from about 10to about 60 degrees and a distance of from about 5 to about 13 cm abovea support which is adapted to rotate light curable denture makingmaterial.

Preferably the product formed in accordance with the invention have amedical application. Most preferably the product formed in accordancewith the invention is a denture, bridge, inlay or other dentalprosthesis or hearing aid.

Preferably the motor is adapted to rotate the substantially horizontalupper surface of the table around a vertical axis of rotation. The lampis positioned about 5 to about 13 cm from the center of the uppersurface of the table. The filament is positioned above the center of thetable an angle of from 10 to 45 degrees to the vertical axis ofrotation.

A comparison is provided in the table below of typical temperatures andirradiant powers obtained using a prior art three lamp (200 watttungsten halogen bulbs) light curing denture making apparatus (a TriadII) and a one lamp (250 watt tungsten halogen bulb) light curing denturemaking apparatus of the invention (a Triad 2000). The height of the topsurface of the table in each apparatus is positioned where its centerreceives maximum irradiance. In this position the distance from the topof an about 0.2 inch thick disk of light curable material placed on thecenter of the table to the closest point on the filament is about 4 and1/8 inch for the Triad II and about 4 and 1/4 inch for the Triad 2000.The table is operated at 6 revolutions per minute. Each apparatus isconnected to a 115 volt source. The temperature of the disk in eachapparatus is measured after 10 minutes of illumination. The irradiantpower during illumination is measured on the upper surface of each diskusing a radiometer with a light guide and a 400 to 500 nanometer bandpass filter.

                  Table                                                           ______________________________________                                                       Triad II                                                                      (Prior Art)                                                                           Triad 2000                                             ______________________________________                                        Disk Temperature                                                              (°F.)     149       155                                                (°C.)     65        68.3                                               Irradiant        93.5      62.4                                               power                                                                         (milliwatts per                                                               square centimeter)                                                            ______________________________________                                    

As shown in the Table above the invention typically provides highertemperatures for curing with about fifty percent less irradiant power.The advantages of the invention include reduced power consumption andreduced equipment cost while at least maintaining the curing temperatureprovided by prior art light curing apparatus. Additionally, apparatus inaccordance with the invention (Triad 2000) provide superior curing ofurethane dimethacrylate resin with visible light activator, noteClinical Research Associates Newsletter Volume 14, Issue 10 October 1990paragraph 2A, the disclosure of which is incorporated herein byreference. The modulus of rupture and compressive strength of productsproduced in an apparatus in accordance with the invention (Triad 2000)is superior as shown in Bangerter et al, J. Dental Research 70: AbstractParagraph No. 2368, 1991, the disclosure of which is incorporated hereinby reference.

OPERATION

A moldable denture base putty-paste that is hardenable with visiblelight is formed according to U.S. Pat. No. 4,551,486 column 10, lines15-68 and column 11, lines 1-2. The putty is pressed into a sheet andthen adapted as a baseplate to a stone model (coated with separator)made from an impression of the mouth. The baseplate is trimmed and thencured for 2 minutes on the rotating table within the housing of anapparatus in accordance with the preferred embodiment of the inventionshown in FIGS. 1-4: the TRIAD 2000 (available from DentsplyInternational: the assignee of the present patent application) under anunfiltered 250 watt tungsten halogen lamp: Dentsply International partnumber 70143. The electromagnetic radiation incident to the surface ofthe baseplate is about 80 milliwatts per square centimeter.

A 0.25 inch diameter rope of putty is adapted around the ridge of thebaseplate in configuration to receive a pressed-in full arch of upperteeth formed as described at column 11, lines 50-55 of U.S. Pat. No.4,551,486. A brush is used to apply bonding agent formed as described incolumn 11, lines 19-47 of U.S. Pat. No. 4,551,486 to an arch of teeth tocover the ridge lap areas and about 2 mm onto the facial, lingual collarand interproximal areas. After setting two minutes the bonding agent isthen light cured in the TRIAD 2000 for two minutes as described above.

The arch of teeth is then pressed into the rope of putty on thebaseplate, and then light cured in the TRIAD 2000 for two minutes asdescribed above. An additional portion of putty rope is used to finishthe facial and lingual aspects of the denture.

Liquid oxygen barrier is formed as described in U.S. Pat. No. 4,551,486column 12, lines 12-28 and coated onto the top of the denture. Thedenture is cured for four minutes in the TRIAD 2000 as described above.The denture is then removed from the stone model and the other side ofthe denture is coated with liquid oxygen barrier and cured for twominutes in the TRIAD 2000 as described above. The denture is then washedin tape water and dried with a paper towel.

Further disclosure of the operation of apparatus in accordance with theinvention is provided in Triad VLC system Technique Manual andOperation/Service Manual for Use With the Triad 2000 and Triad CuringUnits, 1990, distributed by Dentsply International Inc. 570 West CollegeAvenue, P.O. Box 872 York, PA 17405-0872, and incorporated herein byreference in its entirety.

Modulus of rupture, deflection at break, and elastic modulus as usedthroughout this disclosure is measured using ASTM D790 Transverse loadedbeam. Izod impact as used throughout this disclosure is measured usingASTM D256.

Preferably highly filled products (having at least 25% inorganic filler)formed in accordance with the invention have a modulus of rupture of atleast 6,000 psi, a deflection at break of at least 0.1 inch, an Izodimpact of at least 0.7 ft-lb/in, and a depth of cure of at least 2millimeters. Preferably, the products formed in accordance with theinvention (having less than 25% inorganic filler) have a modulus ofrupture of at least 11,000 psi, a deflection at break of at least 0.12inch, an Izod impact of at least 1.3 ft-lb/in, and a depth of cure of atleast 2 millimeters.

Preferably the width, depth and height of the light curing chamber inwhich light curable material is enclosed in accordance with theinvention are each at least 2 inch. More preferably, the width, depthand height of the light curing chamber in which light curable materialis enclosed in accordance with the invention are each at least 3 inch.

When the product and light source are in relative motion, preferably thelight source is positioned above the product at an angle of from 10 to45 degrees to the relative rotation of the product or the light source.

While in accordance with the patent statutes what is at presentconsidered to be the preferred embodiment of the invention has beendescribed it will be obvious to those skilled in the art that numerouschanges and modifications may be made therein without departing from theinvention, and it is therefore intended in the appended claims to coverall such equivalent variations as fall within the spirit and scope ofthe invention.

What is claimed is:
 1. An apparatus to polymerize light curablematerials, comprising:a housing, and a light source, said light sourcebeing supported and enclosed by said housing, said light sourceconsisting essentially of a bulb enclosing a gas and a filament, saidgas comprising a halogen, said filament providing sufficientelectromagnetic radiation to maintain the temperature of light curablematerial positioned within said housing at between 120° and 250° F.(48.9° and 121.1° C.) after at least 2 minutes with at least 50milliwatts per square inch of incident light to substantially evenlycure said light curable material while rotating said light curablematerial.
 2. The apparatus of claim 1 wherein said housing furthercomprises a housing wall having at least one vent allowing air to passout of said housing.
 3. The apparatus of claim 1 further comprising afan, said fan being supported by said housing.
 4. The apparatus of claim1 wherein said filament comprises tungsten, and said light source ispositioned at not more than one point within said housing, and saidlight source further comprises a halogen gas.
 5. The apparatus of claim1 wherein said light source is operable using at least 200 watts ofpower to emit electromagnetic radiation, and provide at least about 50milliwatts per square centimeter at a distance of about 10 cm.
 6. Theapparatus of claim 1 further comprising rotating means and a supportmember, said rotating means being connected to said support member, saidsupport member being rotatable at a constant rate of rotation of fromabout 5 to about 25 revolutions per minute.
 7. The apparatus of claim 1further comprising rotating means and a support member, said rotationmeans being connected to rotate a substantially horizontal upper surfaceof said support member around a vertical axis which intersects a centralpoint on said surface, said filament being positioned along a lineextending through said central point at an angle of from 10 to 45degrees to said axis, said filament being about 5 to about 13 cm fromsaid central point, said filament being positioned above said supportmember.
 8. The apparatus of claim 1 wherein said housing furthercomprises a door having a reflective inner surface.
 9. The apparatus ofclaim 1 wherein said housing further comprises a door, said doorcomprising an optical filter which selectively reflects or absorbselectromagnetic radiation having wavelengths less than 400 nonometers.10. The apparatus of claim 1 wherein said housing further comprises achamber wall, said chamber wall having a reflective surface.
 11. Theapparatus of claim 1 wherein said filament comprises tungsten.
 12. Themethod of claim 1 wherein said radiation is provided without substantialfiltering.
 13. A method to polymerize light curable materials,comprising:providing a housing and a light source, said light sourcebeing supported and enclosed by said housing, said light sourceconsisting essentially of a bulb enclosing a gas and a filament saidfilament using at least 200 watts of power and providing electromagneticradiation, supporting light curable material within said housing, saidmaterial comprises particles of crosslinked polymer and acrylic or loweralkyl acrylic acid ester monomer and projecting sufficientelectromagnetic radiation from said light source onto said materialwhile rotating said material to cause substantial hardening of saidmaterial to form a product.
 14. The method of claim 13 wherein saidfilament comprises tungsten.
 15. The method of claim 13 wherein saidradiation is substantially unfiltered.
 16. A method to polymerize lightcurable materials, comprisingproviding a housing, a light source, andlight polymerizable material, said light source consisting essentiallyof a bulb enclosing a gas and a filament, said gas comprising halogen,said polymerizable material having the form of a dental appliance,supporting said light source and said polymerizable material within saidhousing, said material comprising particles of crosslinked polymer andacrylic or lower alkyl acrylic acid ester monomer and radiating lightfrom said light source onto said polymerizable material while rotatingsaid material, whereby said polymerizable material substantiallycompletely polymerizes in less than 10 minutes to form a denture base,reline, tray, dental appliance or dental restorative product.
 17. Themethod of claim 16 wherein said filament comprises tungsten.
 18. Themethod of claim 16 wherein said radiation is substantially unfiltered.19. A method to polymerize light curable materials, comprising:providinga housing and a light source, said light source being supported andenclosed by said housing, said light source consisting essentially of abulb enclosing a gas and a filament, said gas comprising halogen, saidfilament using at least 200 watts of power and providing electromagneticradiation, supporting light curable material within said housing, saidmaterial comprising particles of crosslinked polymer and acrylic orlower alkyl acrylic acid ester monomer and projecting sufficientelectromagnetic radiation from said light source onto said materialwhile rotating said material to cause substantial hardening of saidmaterial in less than 10 minutes to form a product having an Izod impactof at least 1.3 ft-lb/in when measured using ASTM 0256, and a modulus ofrupture of at least 6000 psi and a deflection at break of at least 0.1inch when measured using ASTM D790 transverse loaded beam.
 20. Themethod of claim 19 wherein at least 200 watts of electrical power isapplied to said filament.
 21. The method of claim 19 wherein saidfilament comprises tungsten.
 22. The method of claim 19 wherein saidlight is substantially unfiltered.
 23. A method to polymerize lightcurable materials, comprising:providing a housing, a light source, andlight polymerizable material, said light source consisting essentiallyof a bulb enclosing a gas a filament, said gas comprising halogen, saidpolymerizable material having the form of a dental appliance, supportingsaid light source and said polymerizable material within said housing,said material comprising particles of cross-linked polymer and acrylicor lower alkyl acrylic acid ester monomer, and radiating light from saidlight source onto said polymerizable material while rotating saidmaterial, whereby said polymerizable material substantially completelypolymerizes to form a cured product.
 24. The method of claims 13, 16,19, or 23 wherein said housing further comprises a support member androtation means, said rotation means being connected to rotate asubstantially horizontal upper surface of said support member around anaxis of rotation, said filament being positioned about 5 to about 13 cmfrom said horizontal upper surface, said light source and said supportmeans being supported by said housing, said light source and saidsupport means being enclosed by said housing, said filament beingpositioned above said support means at an angle of from 10 to 45 degreesto said axis of rotation, said light source providing a power density onsaid upper surface of at least about 50 milliwatts per squarecentimeter.
 25. The method of claims 13, 16, 19, or 23 wherein saidfilament comprises tungsten and said light source further comprises ahalogen gas.
 26. The method of claims 13, 16, 19, or 23 wherein saidlight source is positioned above said polymerizable material, andwherein a vertical axis passes through said polymerizable material, andsaid vertical axis does not intersect said light source.
 27. The methodof claim 13, 16, 19 or 23 wherein said polymerizable material supportedin said housing comprises acrylic or lower alkyl acrylic acid estermonomer, and said polymerizable material comprises particles ofcrosslinked polymer said particles being swollen in said monomer attemperatures not less than 39 degrees centigrade.
 28. The method ofclaim 13, 16, 19 or 23 wherein said polymerizable material is irradiatedwith said radiation at less than 70 milliwatts per square inch for atleast 2 minutes and has a surface temperature of at least 150° F. 29.The method of claim 13, 16, 19 or 23 wherein said dental applicance is adenture, bridge or dental prosthesis.
 30. The method of claim 13, 16, 19or 23 wherein said product formed by said irradiating has a modulus ofrupture of at least 6,000 psi, a deflection at break of at least 0.1inch when measured using ASTM 0790 transverse loaded beam, a Izod impactof at least 0.7 ft-lb/in when measured using ASTM 0256, and a depth ofcure of at least 2 millimeters.
 31. The method of claim 13, 16, 19 or 23wherein said product formed by said irradiating has a modulus of ruptureof at least 11,000 psi and a deflection at break of at least 0.12 inchwhen measured using ASTM 0790 transverse loaded beam, and an Izod impactof at least 1.3 ft-lb/in when measured using ASTM 0256, and a depth ofcure of at least 2 millimeters.
 32. The method of claim 13, 16, 19 or 23wherein said product formed by said irradiating is cured for less than10 minutes and has a modulus of rupture of at least 6,000 psi and adeflection at break of at least 0.1 inch when measured using ASTM 0790transverse loaded beam, and a Izod impact of at least 0.7 ft-lb/in whenmeasured using ASTM 0256, and a depth of cure of at least 2 millimeters.33. The method of claim 13, 16, 19 or 23 wherein said product formed bysaid irradiating is a denture base, reline or tray cured for less than10 minutes and has a modulus of rupture of at least 6,000 psi and adeflection at break of at least 0.1 inch when measured using ASTM 0790transverse loaded beam, and a Izod impact of at least 0.7 ft-lb/in whenmeasured using ASTM 0256, and a depth of cure of at least 2 millimeters.34. An apparatus to polymerize light curable materials, consistingessentially of:a housing (12), circuit means, a light source, and arotatable support means (18), said light source being connected to saidcircuit means, said circuit means comprising a timer (76), a switchmeans, a motor (70), and a fan (86), said switch means being connectedto said timer, filament, motor, and fan, said motor being connected tosaid rotatable support means, said light source being supported andenclosed by said housing, said light source consisting essentially of abulb (14) enclosing a halogen gas and having a filament (15) forproviding sufficient unfiltered electromagnetic radiation to maintainthe temperature of light curable material (16) positioned within saidhousing at between 120° and 150° F. (48.9° and 121.1° C.) after at least2 minutes with at least 50 milliwatts per square inch of incident lightto substantially evenly cure said light curable material.
 35. Theapparatus of claim 34 wherein said filament comprises tungsten.